I'll try to clarify what the issue is. Generally, most square taper cartridge type bottom brackets (BB) are all installed in a similar way.

The crank side or drive side (DS) threads into the frame and seats against the right face of the bottom bracket shell.

On the other side of the bike, the non-drive side (NDS), a cap threads in to the frame and it reaches in far enough to keep the cartridge from wiggling when you pedal. Traditionally these caps held the cartridge in alignment because they fit over the cartridge very closely, like a sleeve.

In the past, you could adjust the exact positioning of your cranks (adjusting the chainline) by adding spacers (say, 1-2 mm) between the DS of the cartridge and the face of the shell. Then you would just thread the adjustable cap on the left side (NDS) a little further in to compensate. Your bottom bracket would be a little further to the right and the chain line would be where you want it. Also, in past BB's the tight fitting NDS cap would hold the cartridge in alignment just because it gripped the cartridge snugly, preventing wiggle.

With the Shimano BB-UN55, the left side (non drive side NDS) cap does not hug the cartridge as tightly as the traditional BB's that came before it. There is a larger amount of space between the cartridge body and the inside of the NDS cap than on previous BB's. The only place this NDS cap comes in tight contact with the cartridge is at the very bottom of the inside of the cap. Unless this cap is socked down tightly over the cartridge, the left side of the cartridge is not being supported. The only support for the cartridge body would be the right side threaded into the frame.

Also, the new BB-UN55 NDS cap has a lip which prevents it from being threaded deeply into the frame. If you were to install spacers to move your crank a little to the right, you may not be able to get the NDS cap screwed in deeply enough to reach the cartridge. In the BB-UN55 the cartridge must reach the bottom of the cap or it will not be held in alignment. The cartridge could then wiggle, maybe loosen up, maybe damage the threads in the bike.

All of the discussions in this string revolve around whether or not this is a design flaw. Those people who have not had problems have tended to dismiss these concerns. I and others who have had problems (or are concerned that problems will arise) are retrofitting an older NDS cap, OR hoping for the best, OR avoiding the product altogether.

I hope this is an accurate and neutral appraisal of the issues and hope it is clearer now.